Try to write it vocabularies everyday and read any thai articles online or listen to thai music. It helps ;)

ankisrs.net This is a really good flashcard program that I use everyday to remember and review new vocabulary words.

As Tak explained, but I'd like to add just a little bit that you should use all words at the same level of formality. For the example, ผม - formal, เธอ - informal, ขอบใจ - informal If you want to make it formal เธอ -> คุณ, ขอบใจ -> ขอบคุณ Informal ผม -> ฉัน/เรา